The present invention relates to a subsystem replacement technique and, more particularly, to a subsystem replacement operation of a central processing unit in an information processing system which is used on the precondition of stopless operation, or the like.
Data migration denotes a migration of data in an old disk subsystem functioning as an external storage in an information processing system to a new disk subsystem.
Generally, as a method of migrating data between disk subsystems, a migration method using intervention of a central processing unit (CPU) is known. In the method, an access from a CPU to a device as a replacement destination is stopped, and the CPU reads data from an old disk subsystem and writes it into a new disk subsystem. According to the method, however, operations of the user to the disk subsystem are stopped for a long time during the data migration.
As techniques which enable the CPU to make an access also during data migration, there are an HODM (Hitachi Online Data Migration) function of Hitachi Limited, an extended remote copy function (hereinbelow, described as "XRC") and a peer-to-peer remote copy function (hereinbelow, "PPRC") ("IBM 3990 Model 6 Enhancements") of IBM Corporation, and symmetric data migration service (SDMS) ("SYMMETRIX ICDA family PRODUCT ANNOUNCEMENT SUMMARY", Nov. 6, 1995) of EMC Corporation.
According to the method of the HODM, an access of the CPU to the old disk subsystem is stopped first. After that, the connection is switched from an access path between the CPU and the old disk subsystem to that between the CPU and a new disk subsystem, and further, an access path is newly provided between the old and new disk subsystems. After that, data of the old disk subsystem is read from the new disk subsystem through the new access path, thereby starting the migration and re-starting the access from the CPU. When the CPU accesses the area from which the data has been migrated, both of the old and new disk subsystems execute processes. When the CPU accesses an area from which the data has not been migrated, data read from the old disk subsystem is reflected also in the new disk subsystem and a process is executed. In this manner, the data migration during an access from the CPU can be realized.
According to the method of XRC, the old disk subsystem has a function of assuring data written from the CPU in a disk controller and the CPU has a function of reading the assured data. By writing the data into the new disk subsystem, the data migration during the access from the CPU can be realized.
According to the method of PPRC, the old and new disk subsystems are connected to each other and are provided with a function of communicating with each other. By writing write data of the CPU to the old disk subsystem to the new disk subsystem by using the communication function, the data migration during the access from the CPU can be realized.
According to the method of SDMS, first, an access from the CPU to the old disk subsystem is stopped. Then, the connection is switched from an access path between the CPU and the old disk subsystem to that between the CPU and the new disk subsystem and, further, a new access path is provided between the old and new disk subsystems. After that, the data of the old disk subsystem is read by the new disk subsystem through the new access path, thereby starting migration. After starting the migration, the access from the CPU is re-started. When the CPU accesses a migrated area, the access is processed directly by the new disk subsystem. When the CPU accesses an area to which the migrating operation has not been executed, data of the track is read from the old disk subsystem and an ordinary process is performed by the new disk subsystem. In this manner, the data migration during the access from the CPU can be realized.
In the above-mentioned methods, by enabling a comprehensive access from the CPU to be performed also during the data migration, stop of an access to the data to be stored into the old disk subsystem can be suppressed to a time for switching from the old disk subsystem to the new disk subsystem. In case of control data of a system such as an OS, even if it is a momentary access stop, it stops the operations of the user and an influence by the migration work is severe. Especially, the access stop is not accepted by the user where 24-hour online operations are necessary. The number of such users is increasing and there is a problem such that the data migration cannot be carried out except for the system stop time such as a winter vacation from the end of the year to the beginning of the next year.
A single subsystem can be used by being connected to a plurality of CPUs. In this case, the subsystem discriminates each of the CPUs on the unit basis of the access path or a group of access paths. An equivalent access to the other subsystem has to be discriminated as well.
When the access path from the CPU is switched to the access path to the new subsystem while making the CPU continuously make accesses, the CPU recognizes that the access is continuously made to the same device. After the data migration is finished and the old subsystem is removed, there is a case that an input request of the device information is issued from the CPU for the purpose of confirmation of the device or the like. In the CPU which confirms the device and the access path in accordance with match/mismatch of device information read in the past with device information read this time. When the information of the new subsystem is sent at this time point, since the device information does not match with each other, it is determined that there is a failure in the access path. Consequently, it is feared that the access path is disconnected and the subsystem becomes down.